The so-called active matrix system in which field effect transistors formed on a substrate are used has been adopted as one system according to which an organic EL panel is driven.
In this system, because the luminance of an organic EL device is controlled with a current, a field effect transistor the mobility of which is so large that a sufficient current can be flowed therein is suitably used.
At present, for example, an amorphous Si semiconductor, a polycrystalline Si semiconductor, or an organic semiconductor has been used in a field effect transistor. However, the amorphous Si semiconductor and the organic semiconductor each have a low mobility. In addition, the polycrystalline Si semiconductor is difficult to increase the area thereof and necessitates a high cost.
Further, the polycrystalline Si semiconductor has grain boundaries, and the characteristics of a device formed thereof will vary owing to carrier scattering, so that a circuit for suppressing the luminance unevenness of the device is separately required to be provided, and the requirement is responsible for an additional increase in the production cost and a reduction in the aperture ratio of the device.
Meanwhile, a system using a conductive oxide as a main component such as a Zn—O system (oxide containing at least Zn) has been vigorously developed as a material for the semiconductor layer of a field effect transistor in recent years (Applied Physics Letters, Vol. 82, pp. 733-735 (2003)).
An oxide semiconductor has a higher mobility than that of the amorphous Si semiconductor, can be formed into a film at a low temperature, and is available at a low cost. Attempts have been made to develop a flexible transistor by forming the oxide semiconductor on a flexible substrate.
In addition, because the oxide semiconductor is transparent to visible light, an improvement in aperture ratio of a device obtained by combining the oxide semiconductor and a light-emitting device can be achieved. The development of an organic EL panel according to an active matrix system obtained by combining a TFT (hereinafter referred to as “TOS-TFT”) using a transparent oxide semiconductor (hereinafter referred to as “TOS”) and an organic EL device has been desired.
Further, an In—Ga—Zn—O system (oxide containing In, Ga, and Zn) (Nature, Vol. 432, pp. 488-492 (2004)) and a Zn—Sn—O system (oxide containing Zn and Sn) are each an amorphous TOS-TFT. In addition, because the amorphous TOS-TFT is theoretically free of carrier scattering which is of concern in a polycrystalline semiconductor, a device using such an amorphous TOS-TFT is expected to have a further increased aperture ratio.
Advanced Materials, Vol. 18, pp. 738-741 (2006) by P. Gorrn et al. discloses a technology for stacking a TOS-TFT of a Zn—Sn—O system (oxide containing Zn and Sn) and an organic EL device on the same substrate.
On the other hand, it has been known that the lifetime of an organic EL device is remarkably reduced owing to the presence of even a trace amount of moisture. In view of the foregoing, the adsorption of moisture is suppressed by covering the entirety of the organic EL device with, for example, a resin or by sealing the device in a layer provided with a moisture adsorbent. Reductions in cost for the production of an organic EL panel and in thickness of the organic EL panel by the sealing of the organic EL device with a solid thin film as a water vapor barrier film will be required to be realized in the future.
U.S. Pat. No. 6,633,121 discloses an organic EL display apparatus in which the water content of each of an organic light-emitting medium, an interlayer insulating film, a color filter, a fluorescent medium, and a planarizing layer is set to 0.05 wt % or less to suppress the generation of a non-light-emitting region, and a method of producing the apparatus.
The production method involving reducing the water content includes performing a dehydration step before and after the formation of an organic light-emitting medium, or alternatively either before or after the formation of the organic light-emitting medium.
The techniques for the dehydration step includes, for example, dew point adjustment, vacuum degree adjustment, inert gas introduction, heat treatment, or a combination of thereof.
The temperature for the heat treatment is desirably 60° C. or more at which the dehydration efficiency is not remarkably reduced, and is desirably 300° C. or less at which an organic film such as an organic light-emitting medium or a fluorescent medium is not thermally damaged.
The period of time for which the heat treatment is performed is influenced by the area and film thickness of a color filter, a fluorescent medium, an interlayer insulating film, or the like but is preferably within the range of 10 minutes to 12 hours. The reason for this is that a dehydration time of less than 10 minutes results in an insufficient dehydration treatment, so that it may be difficult to reduce the water content of an organic light-emitting medium after the assembly to 0.05 wt % or less, and further that a dehydration time in excess of 12 hours merely lengthens the time period for the heat treatment and the effect exerted by more than 12 hours of heat treatment may be not different from that exerted by 12 hours or less of heat treatment.
In view of the foregoing, the dehydration time is more preferably within the range of 30 minutes to 10 hours, or still more preferably within the range of 1 to 6 hours.
In addition, Japanese Patent Application Laid-Open No. 2006-080495 discloses a technology for removing moisture adsorbing to the inside or surface of an insulating film by performing a heat treatment under the atmospheric pressure at 200 to 350° C. and under a reduced pressure at 200 to 400° C., preferably 250 to 350° C. before the formation of an organic layer. Polycrystalline Si is used for a semiconductor layer.
In the case where an insulating film is used as a water vapor barrier film, when a resin film having low heat resistance or the like is used as a flexible substrate or when an attempt to reduce the cost for a production process is to be made, the water vapor barrier film needs to be formed by a lower temperature process.
However, the water vapor barrier property of an insulating film tends to be reduced owing to the film formation at the lower temperature, with the result that a sufficient effect is hardly obtained. The reason is considered that when film formation is performed by a low temperature process (for example, 300° C. or less), it becomes difficult to form a dense film.
In addition, because the respective constituent layers need to be formed at a low temperature of 300° C. or less owing to constraints by the heat resistance of the substrate, the amount of moisture adsorbing to the inside or surface of each constituent layer of a TFT becomes large as compared to that in the case where the layer is formed at a high temperature.
On the other hand, an oxide generally has higher polarization property than that of amorphous Si or polycrystalline Si, so that there is a tendency for the oxide to be extremely liable to adsorb moisture. The tendency becomes remarkable when the film formation is performed at a low temperature. For example, an element showing high polarization property when turned into an oxide like Indium (In) used in the oxide semiconductor in the present invention shows a strong tendency to have high hygroscopic property, so that the amount of moisture adsorbing to the surface thereof is extremely large as compared to that in the case of an amorphous Si semiconductor or a polycrystalline Si semiconductor. Further, when the semiconductor layer is formed of an oxide, the layer contains a large amount of OH groups in the film itself, in addition to the component as water molecules adsorbing to its surface. Because the OH groups can be desorbed as water molecules at the time of heating, merely reducing the content contained in the form of water molecules in the film is not sufficient for the prevention of the degradation of an organic EL device. That is, in the case of an oxide semiconductor layer, it is difficult to prevent the degradation of an organic EL device effectively by merely managing the amount of moisture (component existing in the form of H2O in the layer) in the layer.
U.S. Pat. No. 6,633,121 discloses a technology for suppressing the generation of a non-light-emitting region in an organic EL display apparatus by setting the water content in each of an organic light-emitting medium, an interlayer insulating film, a color filter, a fluorescent medium, and a planarizing layer to 0.05 wt % or less. However, when an oxide semiconductor is used as the semiconductor layer of a field effect TFT as is the case with the present invention, a large amount of a component is desorbed or diffused as H2O from the semiconductor layer after the formation of a device, which poses a problem in the longer term. That is, in the case of a film such as of an oxide semiconductor in which OH groups or the like are present, the groups can be desorbed in the form of H2O at the time of the desorption, so that even when the amount of a component existing in the form of H2O in the film is small, the groups may substantially have adverse effects comparable to those in the case where the film contains a large amount of moisture in itself. Therefore, it is considered that even when the production method disclosed in U.S. Pat. No. 6,633,121 is applied to an organic EL display apparatus having an oxide semiconductor, because a component that can be desorbed as H2O with the elapse of time may be present in the film, the application is insufficient to prevent the degradation of the organic EL device over a long period of time.
A possible method of preventing the degradation of an organic EL device due to a component that can be desorbed as H2O from a TFT using an oxide semiconductor (hereinafter referred to as “OS-TFT”) at the time of heating of the OS-TFT is a method involving forming a film having higher water vapor barrier property between the OS-TFT and the organic EL device.
A water vapor permeability required for a water vapor barrier film is said to be less than 10−5 g/m2/day.
Examples of the film that can satisfy the above-mentioned requirement include a multilayer film obtained by stacking an acrylic resin and aluminum oxide or silicon oxide several times, or a film obtained by alternately stacking SiNx formed into a film by plasma CVD and a plasma-polymerized CNx:H film. However, the films are each formed by a complicated film formation process and necessitate a high production cost.
Accordingly, in order to form an OS-TFT and an organic EL device (typically on the same substrate) by a low temperature process and to give a product having a sufficient lifetime, provision of a costly water vapor barrier layer has been inevitable.